Apparatus for communication systems



Sept. 18, 1945. w, 1 KONRAD APPARATUS FOR COMMUNICATION SYSTEMS Filed March 26, 1943 m v, SK @uw mm n |l|||-l l I J m N. M w f w n i w uw mw uw @B www mm A w Si@ W .a o w w l mw .um H vmnll IIIWN ull.- y. I wwl Nm.; www m Vm. :Sv WfmN www w mm .w m MS m we vw@ "1% NQ M .wm www L. Q 1- MUIIIJ WN l.. L E@ WE w :mw M T IIN@ mm n W NM Patented Sept. 18, 1945 APPARATUS FOR CONIMUCATIO SYSTEMS WilliamL.` Konrad,"1ittsburgh, Pa.,y assignor to The Union Switch'& Signal Company, Swissvale, Pa., a corporation of Pennsylvania.

Application March 26, 194,3, serial Nt. 480,645 7 Claims. (c1. 25o- 210) My invention-.relates to apparatus for communication systems, and more particularly to receiving apparatus for railway train telephone systems.

Train telephone systems using the Itrafic rails in the transmitting channel requirean electrical amplifier of relatively high gain between the train carried receiving inductor and the train carried signaling device such as, for example, a loud speaker. Preferably, the receiving apparatus of such a train communication system remains normally in the circuit ready to receive a message. This normal receiving condition results in amplification of minute earth currents, stray magnetic fields and similar random energy during non communication periods and noise may occur at the loud speaker. Also during communication periods amplification of the noise energy may impair the intelligibility of incoming messages. Furthermore, the energy level of the communication current at the receiving station may vary greatly due to the changing conditions of the "transmitting channel through the rails and the changing distances between the station on a moving train and the remote station with which communication is being held, and as a consequence vof such variations of the energy level, an adjustment of the receiving apparatus is required if uniform level of reproduction at theloud speaker is to be obtained.

In view of such circumstances, one objectof my invention is the provision of communication receiving apparatus incorporating novel and improved means for limiting the amplitude of the voltage applied to a loud speaker.

Another object of my invention is the provision of receiving apparatus of the type here involved incorporating improved automatic volumel control means.

Again, a more specific object of my invention is the provision of an improved peak voltage limiter and of a novel automatic volume control device for receiving apparatusof a railway train telephone system.

Other objects, features and advantages of my invention will appear as the specification progresses.

I shall describe one form of apparatus embodying my invention, and shall then point out the novel features thereof in claims.

According 'to my invention I provide the final tube in a novel manner to limit the peak voltage of the energy applied to the grid circuit of this tube and to control in turn the voltage applied to the loud speaker connected to the output side of this final stage ampliiier. The peak voltage that can be applied to the final stage tube is preselected to be just above the desired communilcationzcurrent Voltage peak so that the limiter does not aiect communication current of normal value but does suppress strong momentary noise impulses and also 'communication current of high values. This peak voltage limiter preferably includes two asymmetric units and a bias voltage means. The bias voltage means includes a resistor interposed between the cathode of the .iinal stage tube and the negative or ground terminal of the plate circuit current source, such resistor being provided with intermediate terminals from which different preselected voltages can be obtained due to the voltage drop through the'resistor caused by the plate current iiowing therethrough. One rasymmetric unit is connected to the grid circuit of this iinal stage tube and to preselected ones of the intermediate terminals of the resistor and is positioned to pass the values of the positive half cycle of noise lenergy and of the communication current voltage greater than the bias voltage obtained from the terminals of the resistor .preselected for this unit. The second asymmetric unit is connected to the grid circuitand to preselected. other ones of the intermediate terminals of theresistor and is positioned to pass the values of the negative half cycle of noise energy and of the communication current voltage greater than the bias voltage obtained from the intermediate terminals of the resistor preselected for this second unit. Thus ,voltages greater `than `the desired peak voltage ,for the communication current are shunted through one or the other or both of the asymmetric units. 1 v i Also, according to myinvention, I provide an automatic volume control means which preferably includes av twin triode electron tube. A

voltage dueto the communication current is takenfrom a preselected .point in the receiving apparatus and fed through a resistance-capacitance ilter to the control grid of one section of the twin triode tube to amplify such voltage. The

4other section of the twin triode tube is arranged as a rectiiier to whose cathode a bias voltage is applied. The output of the amplifying section -of the tu-be is fed to the rectifying section which amplified voltage The energy thus becomes conductive when the overcomes the bias voltage.

:rectied is applied to one or more of the earlier stage tubes ofthe receiving apparatus as a negative bias voltage to control the amplification gain. A unit consisting of a condenser and a resistor is included in the circuit of the automatic volume control means to form a time delay device which maintains the control Voltage between syllables. and words of the4 received communication current. 'g Y For a better understanding of the invention, reference may be had to the accompanying drawing which is a diagrammatic view showing one form of apparatus embodying unyzinventicn when used With receiving apparatus for a railway train.

communication system using a suppressed car-V It; to be understood, however, thatjtheinvention rier single side band telephone currentA is not limited to railway train communication systems and this one application serves to illustrate the many places Where such apparatus is. useful.

Referring tothe drawing, the. receiving appara-tus includes areceiving inductor, two: stages of intermediate frequency ampliicationt a demodulator, a lter, one stage of audio frequency amplification and a loud speak-er. They reference character IC indicates a receiving inductor for picking up energy due to current sent out from aremote station, not shown. In` railway train communication systems of the type referred to here-inbefore, the receiving inductor IC Would be mounted on a train in inductive relation to the rails, the rails bei-ng included in. a transmitting circuit to which cu-rrent is supplied at the remote station.

The first stage of amplification includes an electron tube VIV which may be any one of the several well known -types and is shown as an indiand secondary winding 6 of transformerl Tl is .fi-

connected across control grid T and cathode 8 of tube Vl through two condensers 9 and I0 in series. Preferably; the junction terminal of condensers 9 and l0- is connected to a ground electrode Il and one terminal of primary winding 3 of transformer TI is connected to a ground electrode |2. Direct current of proper voltage for theY plate circuit of tube Vl, as well as for other electron tubes of the apparatus to be referred to later, is supplied from a generator G. When the receiving apparatus is mounted on a train, generator G may be driven by a motor M energizedfrom the usual train lighting source. The plate circuit for tube Vl can be traced from positive terminal B300 of generator G Athrough wire I3, resistor I4, plate l5 and intervening tube space to cathode 8 of tube VI., resistor I6 and ground electrodes l1 and t8 to negative terminal' N300 of generator-G. A first auxiliary grid I9 of tube Vl' is connected to cathode 8' and the other auxiliary grid '20 ofthe tube is supplied with positive voltage from generator G .through resistor 2 l..

It isclear that the electromotive force picked up by inductor IC' due to the communication current, is amplified in the plate circuit of? tube VIv in the Well-known manner. As-an aid to thev understanding of the operation of the appara-tus, I- shall assume that the communication current is a suppressed carrier single side band .telephone current whose carrier frequency is of the order of 5700 cycles per second, the Voice frequency band for modulating the carrier is of 400 to 2500 cycles per Second, and the upper side band of 6100 to 8200 cycles per second is the side band selected for transmission. Thus the plate circuit current.v of tube Vl has acomponent corresponding-to such communication." current received through inductor IC.

The second stage of amplification includes an electron tube Whose control grid circuit is coupled to, the plate circuit of the rst stage tube Vl through a condenser 22. This second stage f of amplification is shown conventionally for the f sake of simplicity because it can be of any wellknown construction and may be substantially a duplication of the first stage, and a description of the second stage is not required for a full understanding of my invention. It is suihcient to point out that the communication current component passed from the plate circuit of tube VI to the contro1 grid circuit ot the second stage is amplified in the usual manner and is supplied from the output of the second stage amplifier to the; input side of a demodulator DM through Wires23' and 24.

' Demodulator DM may be anyone of several well-known forms andv is shown conventionally in order to not undulyl complicate the drawing because its specific construction forms no part of my invention. For example, this demodulator may be of the balanced type-'covered by Letters Patent of the United States No. 2,129,313, granted September 6, 1938' to L. D. Whitelock, for Apparatus for electric signaling systems. It is sufficient forthe present application to point out thatr in demodulator DM a carrier from a local oscillator OS is mixed with the side band vtelephone current suppliedl through the second stage amplifier, and the carrier is suppressed at 4a; push-pull arrangement of primary winding 2.5

of atrans-ormer T2 included in the output circuit of the demodulator. Thus one product of demodulation to. appear in primarywinding 25 is the voice frequencies of the communication current received: by inductor IC.

Band pass filter BPli1 comprises transformer T2, condensers 26 and 21', inductance 28' and at least a portion'of winding 29- of an: auto transformer T3. This band pass lter is arranged in a well-known manner and isproporti'oned to pass the voice frequencies andi tosubstantially eliminate' the other products of demodulation so that the voltages appearing across winding 29 of transformer T3' are those of the voice'frequencies, or atleast of a frequency Within .the selected Voice frequency range.

The final and audio amplin'cation. stage of the receiving apparatus includes an indirectly heated pentode V5, whoseV vfilament. is constantly heated and whose control gridv 301 and cathode 3l are connected tof winding 29A or the band' pass illter through a grid circuit to appear shortly. Plate voltage for plate 32 of tube V5' is supplied from generator G through primary winding 33 of an output. transformer T4 to whose secondary winding 34 a loud speaker LS is connected. It follows that energy passed by the band pass lter .BPF' is amplied by tube' V5 and' is applied to the loud speaker.

./.ccording'to my invention, a peak voltage limiter vis, connected to the naly stage ampliiier vto limit the peak voltage applied to the control grid of the tube of the finali stage. and in turn to control the peak voltage that can vbe applied to the loud speaker LS. This voltage limiter VL may take diierent forms, and preferably includes an electron tube V6 and a resistor R. Resistor R is interposed in the lead between cathode 3| of tube V and the negative terminalN39l of generator G, the connection being completed through ground electrodes 35 and I8. Resistor R is thus interposed in the plate circuit of tube V5 and a voltage drop is created across the resistor due to the plate current, the value of such Voltage drop being predetermined according to the value of resistor R and to the value of the plate current. Resistor R is provided with a negative terminal Z and two adjustable intermediate or mid terminals X and Y. Thus the voltage of terminals X and Y with respect to the negative terminal Z and with respect to each other can be preselected by their positions. For example, 4terminal Y Amay be four volts positive with respect to terminal Z and terminal X may be four Volts positive with respect to terminal Y, and in turn eight volts positive with respect to terminal Z.- Electron tube V5 is preferably a twin triode, that is a tube containing two triodes in a common envelope. Each triode of tube V6 is preferably converted into an asymmetric unit or one-way thermionic Valve connecting the grid and plate together. I have found that a twin triode tube thus arranged gives more satisfactory operation becauseof the lower tube impedance than a twin diode or `two separate diodes. My invention is not limited, however, to a twin triode-arranged to form two one-way thermionic valves in the manner here used and other forms of tubes can be used. The filaments of tube V5 are constantly heated and for convenience I shall refer to the left-hand side of the tube as viewed in the drawing as section'l of the tube and to the right-hand side of the tube as section 2.

Each section of thetube V6 is connected across thegrid circuit of tube V5 through a portion of resistor R. Specifically, the control grid circuit for tube V5 can be traced from control grid 30, through condenser 35, winding 29 of transformer T3, wire 31, terminal Yvof resistor R, and top portion of resistor R and wire 38 to cathode 3|, a resistor 39 being connected between control grid 39 and aground electrode 4I) and a condenser 4l being connected between cathode 3I and ground electrode 40. Section l of tube V6 is connected to this grid circuit byhaving its plate or anode 42 connected to the top terminal of winding 29 by wire 46 and its cathode 43 connected to the lower terminal of winding 29 through terminal Xof resistor R, theY portion of resistor R to terminalY and wire 31. It is clear that this first section is provided with a bias voltage equal to the' voltage drop between terminals X and Y, and its cathode 43 is positive in potential with respect to its anode 42. ASection 2 of tube VB is connected to the grid circuit of tube V5 by having its-anode 44 connected to the lower terminal of winding 29 through negative terminal Z of resistor R, that portion of resistor R to terminal Y and wire 31, and its cathode 45 connected to the top terminal of winding 29 through wire v46. Thus this second section is provided with a bias Voltage equal to the voltage drop between terminals Y and Z of resistor R, the cathode 45 being positive in potential with respect to anode 44. The positions of terminals X and Y are preselected so that the voltages of these terminals are such that each section of tube V6 is provided with a bias voltage that is only slightly greater than the'desired peak voltage of the communication current appearing in winding 29 and applied to tube V5. Forexample, each such bias voltage may be of the order of four volts. L

It can be seen therefore that any electromotive force created across winding 29 of transformer T3 to be applied to the control grid of tube V5, would be shunted through one or the other of the sections of tube V6 of the voltage limiter but for the bias voltages applie'd to the sections. For example, when the electromotive force appearing in winding 29`causes the top terminal ofthe winding to be positive, section I of tube V6 tends to conduct the current but current ow is blocked by the bias voltage derived from terminal X of resistor R. This section becomes conductive however as soon as the electromotive force is greater than the bias voltage. Similarly, when the electromotive force of winding 29 `causes its lower terminal to be positive the section 2 of tube V5 tends to conduct the current but flow of current is blocked by the bias voltage derived from terminal Y, this section becomingconductive when the electromotive force is of a value suiiicient to overcome the bias Voltage. Consequently, the values of the positive half cycles of the communication current higher than a desired peak value are shunted from the grid 'circuit of tube V5 by one yasymmetric unit or oneway thermionic valve of the voltage limiter, and the values of the negative halfrcycles of such current higher than the desired peak value are shunted by the other asymmetric `unit or oneway thermionic valve of the voltage limiter. Likewise, the values of an impulse of noise energy higher than the predetermined value appearing across winding 29 are shunted by the voltage limiter and these higher voltages of the noise energy are prevented from appearing at the loud speaker. Hence noise energy occurring during non-communicative periods is attenuated so as to not be annoying. It is to be observed that noise energy of sucient strength to be shunted by the voltage limiter occurring during communication periods may cause some shunting of the desired communication current voltage. However, in systems of the type here contemplated, the noise impulses are ordinarily of short duration as compared to the desired communication current voltages, and the desired communication voltage is not affected long enough to impair its intelligibility while the noise is greatly attenuated. It is also to be observed that the component of the plate current of tube V5 corresponding to the communication current is by-passed by condenser 4I, leaving the bias voltages for the voltage limiter to be of substantially the same value during communication periods as is provided during non communication periods. l

An automatic volume control unit AVC is provided for the receiving apparatus. This unit AVC includes a twin triode tube V1, whose laments are heated in the usual manner. Voltage from a preselected terminal B of filter BPF is applied to control grid 41 and cathode 52 of the right-hand triode of tube V1 as viewed in the drawing, the circuit connection including wire 5I, a resistance-capacitance filter comprising resistor 48 and condensers 49 and 59, and a condenser 53. Plate Voltage is supplied to plate 56 of the right-hand triode of tube V1 from generator G through wire I3 and resistor 54, the plate circuit being completed from cathode 52 through ground electrodes 55 and I8 to negative terminal N300 of generator G. ThusA the communication current. voltage appearing at terminal B of the band pass. lter is amplified by the right-hand triode of tube V1 of the automatic volume control: unit.

fil-hef lett-,hand triode of tube V1 is converted intof a. diode rectifier by its. controlz grid 51 being connected to plate 58,: A bias voltage is. applied to cathode 5.5i of the diode rectifier through resistors and El connected across generator: G and a resistor `(5-2 connected between the cathode and a preselected midtermi-n'al of resistor 60.. Plate -58 ot the diode rectier is further connected. to -a ground electrode 55 through resistor 6.3. .'lhusc the cathode. of the diode rectiiier is madel positive in potential with respect to the plate. 58 bya voltage preselected by theconnection; to resistor 6U.. The amplied output` of the triod'e. or4 tube V1. is appliedl to the diode section of the. .tube through. condenser 64 connected. between. plate 53. of the triode. and cathode 5S of the diode. Consequently, when the voltage from terminal B after amplification at the triode of tubeV-1 is greater than the bias voltage of the diode sectionA of the tube, the voltage is rectied i bythe diode and current. flows in resistor 63, the top terminal as viewed .in the drawing of resistor 53v being-negative with respect, to ground electrode 55.

This voltage created across resistor 63 is supplied to the control grid l4 of therststage tube VI. through a. connection including wire 65 and resistor E6. 'Ihus when thev communication current is of a large amplitude and. a relatively large voltage appears at terminal/B of the band pass iilter, the first stage: tubev VI is provided with an additional negative bias voltage to reduce the amplication` gain. If desired, this automatic volume control voltage can be applied to. the tube of the second stage* of the ampliiier by a connection including. wire 68 extending from resistor 66 tothe grid circuit of the second stage tube.

A condenser 6,1 is connected in shunt with resistor 63'r to form a time delay device; condenser lil being charged by theA rectified voltage and such charge serving to maintain the automatic volume control. voltage between syllables and words, of: the received communication current.

Itis to be seen from the foregoing description that I have provided the receiving apparatus with a peak voltagef limiter which shunts communicationcurrent and noisevoltages above a predetermined' value, with the result the loud speaker is comparatively quiet during noncommunication periods and during communication periods excessive loudness of the message is avoided and? noise-s greatly attenuated without affecting the intelligibility of the message. Also the automatic volume control unit permits adjustment of the receiving apparatus. for a relatively low energy level of communication current Without excessive loudness when the communication current becomes of relativelyv high amplitud'e.

Although I have herein shown and described only one form of apparatus for communication systems embodying my invention, it is understood thatY various changes and `modiiications may be. made.- therein within the scope of the appended. claims without departing from the spiritand scope of my invention..

Having thus described my invention, what I claim is:

communication receiving apparatus including an electron tube amplifier provided with a grid circuit having a Winding through which a communication electromotive force is applied and with a platev circuit supplied with current from a. direct current source. and coupled to a loud speaker, the combination comprising, a resistor interposed between the cathode of said tube and the negativeterminalt of said direct current source and having arst intermediatev terminal connected to one .terminal of said grid circuit winding to include the full resistor in said plate circuit and a portion in said grid circuit; a voltage limiter including a rst and second one-way thermionic valve arranged with the. anode of the rst valve andv the cathode of the second valve connected to the other terminal of said grid circuit winding, the, anode of the second valve connected to the negative terminal of said direct currentv source and the cathode of the rst valve connected to a second intermediate terminal of said resistor; and said iirst and sec ondterminals positioned cn said resistor to bias said. iirst valve. by the voltage drop derived loe-y tween said' second` and iirst terminals and to bias said. second valve by the voltage drop derived between said first terminall and said negative terminal.

2. In communication receiving apparatus including an electron tube amplied provided with a grid circuit having a winding through which a communication electromotive force is applied and with a plate circuit supplied with current from a direct currentsource and coupled to a loud speaker, the combination comprising;` a resistor interposed between the cathode of said tube and the negative terminal of said direct current source and provided with a first intermediate terminal to which one terminal of saidv grid circuit winding is connected to include the. full resistor in the plate circuit and a portion in said grid circuit; anda voltage limiter including a first and a second rectifying device each having one of its terminals connected to the other terminal of said grid circuit winding, the other terminal of said first device connected tothe negative. terminal of said direct current source, and the other terminal of said second device connected to. a second intermediate terminal of said resistor more remote from said negative terminal than said first intermediate terminal, and said. rst and second rectifying devices poled to pass current'in opposite directions.

3. In communication receiving apparatus having an audio stage ampliiier including an electron tube provided with a grid circuit including a winding through` which communication electromotive force is received and with a plate circuit supplied with current from a direct current source and coupled to a loud speaker, the combination comprising, a resistor interposed in the plate* circuit between the cathode of said tube and the negative terminal of said direct current source, said resistor having a rst. intermediate terminal to which one terminal of saidgrid circuit winding isconnected, a rst rectifying device having one terminal connected to the other terminal or said grid circuit winding, and its other terminal connected to a second intermediate terminal of said resistor, a second: rectifying device having one terminal, connected to. the negative end ot said resistor and its other terminal connected to said other terminal of said grid circuit winding, andV said rst and second intermediate terminals preselected as to their positions on said resistor tobias saidv first device due .to the voltage drop created in the resistor between said second and first terminals and to bias said second device due to the voltage drop created in the resistor between said first terminal and said negative end, and said rst and second rectifying devices disposed to pass current in opposite directions.

4. In communication receiving apparatus having an audio stage amplier including an electron tube provided with a grid circuit to which a communication electromotive force is applied and with a plate circuit supplied with current from a direct current source and coupled to a loud speaker, the combination comprising, a resistor interposed in said plate circuit between the cathode of said tube and the negative terminal of said direct current source to provide a voltage drop due to the plate current flowing through the resistor, said resistor having a first and a second intermediate terminal arranged in the order named from the negative end of the resistor, said grid circuit connected to said first intermediate terminal, a rst one-way thermionic valve connected across said grid circuit with its cathode connected to said second intermediate terminal, and a second one-way thermionic valve connected across said grid circuit with its anode connected to the negativeend of said resistor.

5. In communication receiving apparatus hav- `ing an amplifying stage including an electron tube provided with a, grid circuit to which a communication electromotive force is applied and a plate circuit supplied from a direct current source and coupled to a loud speaker, the combination comprising, a resistor interposed in said plate circuit between the cathode of said tube and said direct current source; said resistor provided with a first, a second and a third terminal disposed in the order named from the negative end of the resistor and spaced for the second terminal to have a preselected positive voltage with respect to the rst terminal and the third terminal to have the same preselected positive voltage with respect to the second terminal due to the plate circuit current flowing through the resistor; said grid circuit connected to said second terminal to connect that circuit to the cathode of the tube, a first one-way thermionic valve connected across said grid circuit with its cathode connected to said third terminal to shunt the values of the positive half rcycle of current applied to said grid circuit greater than said preselected voltage, and a second one-way thermionic valve connected across said grid circuit with its anode connected to said first terminal to shunt the values of the negative half cycle of current applied to said grid circuit greater than said preselected voltage.

6. In communication receiving apparatus having an amplifying stage including an electron tube provided with a grid circuit to which a communication electromotive force is applied and a plate circuit supplied from a direct current source and coupled to a loud speaker, the combination comprising, a resistor interposed in said plate circuit between the cathode of said tube and said direct current source; said resistor provided with a rst, a second and a third terminal disposed in the order named from the negative end of the resistor and spaced for the second terminal to have a preselected positive voltage with respect to the rst terminal and the third terminal to have the same preselected positive voltage with respect to the second terminal due to the plate circuit current flowing through the resistor; said grid circuit connected to said second terminal to connect that circuit to the cath-` ode of the tube, a first one-way thermionic valve connected across said grid circuit with its cathode connected to said third terminal to shunt the values of the positive half cycle of current applied to said grid circuit greater than said preselected voltage, a second one-way thermionic valve connected across said grid circuit with its anode connected to said first terminal to shunt the values of the negative half cycle of current applied to said grid circuit greater than said preselected voltage, and a condenser connected between the cathode of said amplifier tube and the negative terminal of said direct current source to bypass the communication current component of the plate circuit current around said resistor.

7. In combination, an amplifying electron tube having a plate circuit supplied with current from a source of direct current and including an output winding; a resistor interposed in said plate circuit between the cathode of said tube and the negative terminal of said direct currentsource and provided with a first, a second and a third terminal disposed. in the order named from the negative end of the resistor and spaced for the plate circuit current to create the same preselected voltage drop between the third and second terminals and between the second and first terminals; a grid circuit including a coupling element having one terminal connected to the control grid of said tube and its other terminal connected to said second resistor terminal; and a twin triode having the grid of each of its sections connected to the anode of that section for each section to serve as an asymmetric unit, said triode having the anode of a rst one of its sections and the cathode of a second one of its sections connected to said one terminal of said coupling element, the anode of its second section connected to said rst resistor terminal and the cathode of its rst section connected to said third resistor terminal to limit the voltage applied to the grid 0f said tube through said coupling element to a value not greater than said preselected voltage drop.

WILLIAM L. KONRAD. 

